The present invention is directed to a pressure sensitive switch and, more particularly, to a switch for completing an electrical circuit when a desired fluid pressure level is reached and to a method of setting the switch contact gap.
Various pressure sensing electrical switch arrangements have been used in the past to detect when the pressure of a fluid equals or exceeds a predetermined pressure level. Many switches of this type have a coil spring which opposes the force exerted by the fluid on a piston. When the pressure of the fluid exerts a sufficient force, the piston is moved and ultimately causes electrical contacts to make or break an electrical circuit.
One type of pressure responsive switch arrangement for determining the pressure of a fluid within specified pressure ranges is shown in U.S. Pat. No. 3,786,210, issued Jan. 15, 1974, to Bayam. In the Bayam reference, three switches having springs which differ in spring constant, are provided. Each of the pressure sensitive switches receives pressurized fluid through a separate opening in a covering plate. Each switch is actuatable at a different pressure and has a metal piston which is spring based toward the covering plate. Current is supplied to each piston by means of its associated compression spring. When actuated, the piston of a switch will have been moved against the opposing force of the compression spring so as to contact the metallic casing. A separate resistance is in series with each compression spring and all three resistances are connected to a single electrical connector. It is possible, therefore, to determine the pressure of the fluid, within the ranges specified, by measuring the value of resistance connected between the single connector and the conductive portion of the casing.
Another type of pressure sensitive switch is shown in U.S. Pat. No. 3,573,410, issued Apr. 6, 1971, to Budzich et al. A non-conductive piston in the device of Budzich actuates a bistable snap element upon which are mounted the electrical contacts for the switch. In the Budzich et al device, the pressure level for actuation is determined by the spring constant of a coil spring opposing the motion of the piston. Additionally, this spring carries current in the electrical switch circuit when this circuit is closed.
In U.S. Pat. No. 3,622,976, issued Nov. 23, 1971, to Howard, a pressure sensitive switch is disclosed which includes a spring biased plunger. The plunger normally completes an electrical circuit between a terminal connected to the biasing spring and a conductive portion of the switch case. When sufficient force is applied to the plunger, it is moved against the biasing force of the spring, thus breaking the circuit and indicating the pressure level of the fluid being monitored.
The gap between the electrical contacts in such a switch has, in the past, generally been set by maintaining the dimensional stability of the elements which form the switch. It is desirable to provide a small gap between the contacts such that the movement of the switch elements is minimized and wear on the moving parts, particularly the membrane at the fluid inlet, is reduced. It will be appreciated, however, that setting an especially small contact gap in such a switch requires that the switch elements be manufactured to close tolerances and that this, in turn, adds significantly to the cost of the switch.
There is a need for a simple pressure sensitive switch which is of economical design and which is reliable in operation and for an improved method of setting the contact gap in such a switch.